This invention relates to a method and apparatus for adjusting the intensity profile of an ultrasound beam used in an ultrasound imaging system.
In a conventional ultrasound imaging system an ultrasound beam is generated by multiple piezoelectric crystals which simultaneously radiate ultrasound signals into an object to be imaged. The beam may include side lobes which can cause degradiation of the ultrasound image of the object.
Prior art ultrasound imaging systems have used a technique called apodization to decrease the effect of the beam side lobes and thereby to increase image resolution. In this technique, the ultrasound signals generated by the boundary piezoelectric crystals are attenuated relative to the signals generated by the center crystals. This technique may be implemented by attenuating the drive signals applied to the boundary crystals relative to the drive signals applied to the center crystals. Prior art systems have used individual attenuators for each crystal to produce a desired ultrasound beam profile. Unfortunately, in order to modify the profile, individual attenuators must be replaced in a procedure that is often tedious and time-consuming.
In accordance with the illustrated preferred embodiment of the present invention, an ultrasound imaging system has a beam intensity profile that is simple to modify. One benefit of this flexibility is that the profile may be adjusted as necessary to obtain an improved resolution of the ultrasound image. Instead of adjusting drive signal amplitudes, as is done in the prior art, the system adjusts the duration of the drive pulses. Since each pielectric crystal generates a maximum amplitude signal at its resonant frequency, the signal amplitude may be varied by varying the drive frequency around the crystal resonant frequency. The drive frequency may easily be varied by varying the duration of the drive pulses since the drive frequency is related to the drive pulse duration as may be demonstrated by Fourier analysis.
For example, a train of short drive pulses has a spectrum containing more high frequency components than does a train of longer drive pulses. Since these high frequency components are strongly attenuated by a piezoelectric crystal, the resulting ultrasound signal has a lower amplitude than if the longer drive pulses were used. By adjusting the durations of the drive pulses applied to individual piezoelectric crystals, the intensity profile of the ultrasound beam can be shaped as desired. A desired shaping may involve decreasing side lobe amplitudes in order to maximize images resolution.
An apparatus constructed in accordance with the preferred embodiment includes a drive pulse adjuster for adjusting the durations of the drive pulses. This adjuster may be driven by a single voltage source so that circuit costs may be kept low. The ultrasound system may be a scanner in which successive groups of piezoelectric crystals are activated so that an object may be scanned. In addition, the intensity profile may be controlled by a computer for easy modification. Computer control may be particularly advantageous in compensating for performance variations between individual piezoelectric crystals due to manufacturing tolerances. In addition, computer control allows beam shaping which may be used to vary the depth range of the beam.